1. The Field of the Invention
The present invention relates generally to x-ray tubes. More particularly, embodiments of the present invention relate to an x-ray tube cooling system that increases the rate of heat transfer from the x-ray tube to a cooling system medium so as to significantly reduce heat-induced stress and strain in the x-ray tube structures and thereby extend the operating life of the device.
2. The Prior State of the Art
X-ray producing devices are extremely valuable tools that are used in a wide variety of applications, both industrial and medical. For example, such equipment is commonly used in areas such as diagnostic and therapeutic radiology; semiconductor manufacture and fabrication; and materials analysis and testing. While used in a number of different applications, the basic operation of x-ray tubes is similar. In general, x-rays, or x-ray radiation, are produced when electrons are produced, accelerated, and then impinged upon a material of a particular composition.
Typically, this process is carried out within a vacuum enclosure. Disposed within the evacuated enclosure is an electron generator, or cathode, and a target anode, which is spaced apart from the cathode. In operation, electrical power is applied to a filament portion of the cathode, which causes electrons to be emitted. A high voltage potential is then placed between the anode and the cathode, which causes the emitted electrons accelerate towards a target surface positioned on the anode. Typically, the electrons are xe2x80x9cfocusedxe2x80x9d into a primary electron beam towards a desired xe2x80x9cfocal spotxe2x80x9d located at the target surface. In addition, some x-ray tubes employ a deflector device to control the direction of the primary electron beam.
For example, a deflector device can be a magnetic coil disposed around an aperture that is disposed between the cathode and the target anode. The magnetic coil is used to produce a magnetic field that alters the direction of the primary electron beam. The magnetic force can thus be used to manipulate the direction of the beam, and thereby adjust the position of the focal spot on the anode target surface. A deflection device can be used to control the size and/or shape of the focal spot.
During operation of an x-ray tube, the electrons in the primary electron beam strike the target anode surface (or focal track) at a high velocity. The target surface on the target anode is composed of a material having a high atomic number, and a portion of the kinetic energy of the striking electron stream is thus converted to electromagnetic waves of very high frequency, i.e., x-rays. The resulting x-rays emanate from the target surface, and are then collimated through a window formed in the x-ray tube for penetration into an object, such as a patient""s body. As is well known, the x-rays can be used for therapeutic treatment, or for x-ray medical diagnostic examination or material analysis procedures.
A percentage of the electrons that strike the target anode target surface rebound from the surface and then either impact at other random areas on the target surface, or at other xe2x80x9cnon-targetxe2x80x9d surfaces within the x-ray tube evacuated enclosure. The electrons within this secondary electron beam are often referred to as xe2x80x9csecondaryxe2x80x9d electrons. These secondary electrons retain a significant amount of kinetic energy after rebounding, and when they impact these other non-target surfaces a significant amount of heat is generated. This heat can ultimately damage the x-ray tube, and shorten its operational life. For example, the temperatures generated by secondary electrons, in conjunction with the high temperatures generated by the primary electrons at the focal spot of the target surface, often reach levels high enough to damage portions of the x-ray tube structure. For example, the joints and connection points between x-ray tube structures can be weakened when repeatedly subjected to such thermal stresses. In some instances, the resulting temperatures can even melt portions of the x-ray tube, such as lead shielding disposed on the evacuated enclosure. These situations can shorten the operating life of the tube, affect its operating efficiency, and/or render it inoperable.
Further, because the trajectories of secondary electrons cause them to impact some interior surface locations with relatively greater frequency than other areas, the resulting heat distribution can be uneven. The varying rates of thermal expansion cause mechanical stresses and strains when the cooler part of the structure resists the expansion of the hotter portion of the structure. Ultimately, this can cause a mechanical failure in the part, especially over numerous operating cycles.
While the aforementioned problems are cause for concern in all x-ray tubes, these problems become particularly acute in the new generation of high-power x-ray tubes which have relatively higher operating temperatures than the typical devices. In general, high-powered x-ray devices have operating powers that exceed 20 kilowatts (kw).
Attempts have been made to reduce temperatures in such areas of high heat concentration, and to minimize thermal stress and strain, through the use of various types of cooling systems. However, previously available x-ray tube cooling systems have not been entirely satisfactory in providing effective and efficient cooling, and have been especially ineffectual in those particular regions of the tube that are subjected to high temperatures, such as from rebounding electrons. Moreover, the inadequacies of known x-ray tube cooling systems are further exacerbated by the increased heat levels that are characteristic in high-powered x-ray tubes.
For example, conventional x-ray tube systems often utilize some type of liquid cooling arrangement. In such systems, at least some of the external surfaces of the vacuum enclosure are placed in contact with a circulating coolant, which facilitates a convective cooling process. While these types of processes are adequate to cool some portions of the x-ray tube, they may not adequately cool areas of localized heatxe2x80x94such as those that are susceptible to heating from secondary electrons, including the aperture and window areas of the tube.
In fact, conventional cooling processes are particularly ineffective for cooling the aperture portion, because it presents a limited external surface area with which to effectuate heat transfer to the surrounding coolant. Moreover, the positioning of a deflection mechanism, such as a magnetic coil, further inhibits adequate cooling of the aperture when conventional methods are used. In particular, a magnetic coil (or similar deflection mechanism), is disposed around the periphery of the aperture, so its position preventsxe2x80x94or at least limitsxe2x80x94the amount of heat that can be convectively transferred from the aperture to the surrounding coolant.
In addition to the aperture, the window area of the x-ray tube is also particularly susceptible to heat generated from rebounding electrons due to its proximity to the anode target. In fact, even in relatively low-powered x-ray tubes, the window area can become sufficiently hot to boil coolant that is adjacent to the window. The bubbles produced by such boiling may obscure the window of the x-ray tube and thereby compromise the quality of the images produced by the x-ray device. Further, boiling of the coolant can result in the chemical breakdown of the coolant, thereby rendering it ineffective, and necessitating its removal and replacement. Also, the window structure itself can be damaged from the excessive heat; for instance, the weld between the window structure and the evacuated housing can fail.
In view of the foregoing problems and shortcomings with existing x-ray tube cooling systems, it would be an advancement in the art to provide a cooling system that removes heat from the x-ray tube, and that effectively removes heat from specific regions of the tube, such as the aperture and window portions of the vacuum enclosure. Further, the cooling system should effect sufficient heat removal so as to reduce the amount of thermally-induced mechanical stresses otherwise present within the x-ray tube, and thereby increase the overall operating life of the x-ray tube. Likewise, the cooling system should substantially prevent heat-related damage from occurring in the materials used to fabricate the vacuum enclosure, and should reduce structural damage occurring at joints between the various structural components of the x-ray tube.
The present invention has been developed in response to the current state of the art, and in particular, in response to these and other problems and needs that have not been fully or adequately solved by currently available x-ray tube cooling systems. Thus, it is an overall object of embodiments of the present invention to provide a cooling system that effectively and efficiently removes excessive heat from x-ray tube components.
It is also an object to provide a cooling system that will efficiently and effectively remove heat from specific regions of the x-ray tube that are routinely exposed to particularly high temperatures. Similarly, it is an objective to remove heat at a higher rate from these specific regionsxe2x80x94as opposed to other relatively cooler regionsxe2x80x94so as to maintain a substantially uniform thermal state as between the various x-ray tube regions and avoid destructive thermal expansion discrepancies.
Another related objective is to remove sufficient heat from the x-ray tube as to reduce the occurrence of thermally induced stresses that could otherwise reduce the tube""s operating efficiency, limit its operating life, and/or render the tube inoperable.
In summary, these and other objects, advantages, and features are achieved with an improved cooling system for use in effecting heat transfer from any x-ray tube. Embodiments of the present invention are particularly suitable for use with high-powered x-ray tubes.
In a preferred embodiment, the x-ray tube cooling system utilizes a liquid coolant that is continuously circulated through a coolant reservoir by an x-ray cooling unit. The system also includes a shield structure that is disposed about an exterior surface of the x-ray tube vacuum enclosure, and preferably in a manner such that the shield is substantially adjacent to both the aperture portion and the electron beam deflection device that is disposed around the aperture. In presently preferred embodiments, the shield structure includes an inlet port and an outlet port. The inlet port is in direct fluid communication with the external cooling unit and the outlet port is in fluid communication with the interior of the coolant reservoir.
In operation, the heat is removed from the coolant by the external cooling unit, and the coolant is then supplied directly to the shield structure by way of the inlet port. As the coolant proceeds through the flow passage defined by the shield structure, heat radiated from the aperture portion and the deflection device is absorbed. In a preferred embodiment, the coolant is then discharged into the reservoir from the outlet port. In preferred embodiments, the discharged fluid is directed across the window area of the x-ray tube, so as to enhance the removal of heat from that particular region. Also, since in preferred embodiments all coolant exiting the external cooling unit proceeds directly through the inlet port of the shield structure, heat is removed from the region of the aperture at an increased rate. Moreover, the unique positioning and orientation of the shield ensures adequate heat removal from the aperture portionxe2x80x94even in the presence of the deflection device.